As shown in FIG. 3, a conventionally known machine tool of this type is an internal grinding machine equipped with two work spindle stocks 1 and 2 and wheel spindle stocks 3 and 4 (this type of machine tool is called a simultaneous double spindle internal grinding machine). The work spindle stocks 1 and 2 are provided with revolvable work spindles 100 and 200 arranged in parallel and respectively having chucks 101 and 201 on their tip parts for setting work pieces.
The wheel spindle stock 3 is provided with a first grinding wheel 300 and the other wheel spindle stock 4 is provided with a second grinding wheel 400. The first grinding wheel 300 is placed opposed to the tip of the work spindle 100 and the wheel spindle stock 3 is attached to a built-in rotary shaft with a quill while the second grinding wheel 400 is placed opposed to the tip of the other work spindle 200 and the wheel spindle stock 4 is attached to the built-in rotary shaft with the quill.
Both work spindle stocks 1 and 2 are attached to an index unit 5 for revolving (180.degree.) the work spindles 100 and 200 of both work spindle stocks 1 and 2 against each other and, by this revolving movement, the positions of the work spindles are reversed so that the tip of the work spindle 100, which is initially opposed to the first grinding wheel 300, is placed opposite to the second grinding wheel 400 and the tip of the other work spindle 200, which is initially opposed to the second grinding wheel 400, is placed opposite to the first grinding wheel 300.
The wheel spindle stock 3 is constructed by an SZ table 6 movable in an SZ direction and an SX table 7 movable in an SX direction, and the other wheel spindle stock 4 is constructed by a BZ table 8 movable in a BZ direction and a BX table 9 movable in a BX direction so as to be moved in these directions as indicated by arrows in FIG. 3. Angles for the BX and SX tables 9 and 7 can be optionally set against the BZ and SZ directions.
The internal grinding machine of this type allows simultaneous grindings by the first and second grinding wheels 300 and 400. In particular, by the revolving movement of the index unit 5, after pieces to be worked set on one work spindle 100 are ground by the first grinding wheel 300, they can be ground again by the second grinding wheel 400 without any setting changes, resulting in one chucking work necessary for both grindings. Thus, it is a suitable machine for grinding pieces requiring highly accurate concentricity on two parts to be ground, e.g., a fuel injection nozzle having a hole part and a conical part on its tip.
Further, when the respective work spindles 100 and 200 are expanded by heat generated during grindings by the first and second grinding wheels 300 and 400, a grinding position (the position of the first or the second grinding wheel 300 or 400 against the work spindle 100, or against the other work spindle 200) is changed and the grinding accuracy is adversely affected. Thus, when the work pieces are ground by the first and second grinding wheels 300 and 400, both work spindles 100 and 200 are cooled by spraying coolant on the outer peripheral frames thereof from the upper sides.
In the internal grinding machine of the conventional type mentioned above, a means adopted for cooling both work spindles 100 and 200 is one by which coolant is sprayed on their outer peripheral frames from above. However, this is not a good means to sufficiently cool the work spindles 100 and 200 because not many parts thereof are directly sprayed by coolant, thereby resulting in cooling of only a few areas of the spindles. Moreover, there exist bearings for the work spindles 100 and 200 within their outer peripheral frames and bearings for revolving movement of the index unit therewithin as heat sources which are not sufficiently cooled. For this reason, various problems occur, such as the gradual increase in temperature of the work spindles 100 and 200 leading to the gradual expansion thereof, shifts in the grinding positions (of the first grinding wheel 300 against one work spindle 100, etc.) due to the expansion, and variable and thus unstable accuracy for shaping work pieces with the lapse of time and the like.
The present invention is based on such a background and its object is to provide a machine tool having two revolvable spindles, e.g., an internal grinding machine capable of sufficiently cooling the work spindles and thus providing accuracy for shaping work pieces in a stable manner even with the lapse of time.